Traditionally, color ink jet printing is accomplished by one of two technologies, referred to as drop-on-demand and continuous stream printing. Both technologies require independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the printhead. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited upon a medium. Typically, each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce up to several million perceived color combinations.
In drop-on-demand ink jet printing, such as shown in U.S. Pat. No. 6,065,825, ink droplets are generated for impact upon a print medium using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the printhead and the print medium and strikes the print medium. The energy to propel such droplets from the ejector comes from the pressurization activator associated with that ejector. The formation of printed images is achieved by controlling the individual formation of ink droplets at each ejector as the medium is moved relative to the printhead.
Conventional drop-on-demand ink jet printers utilize a pressurization actuator to produce the ink jet droplet from the nozzles of a printhead. Typically, one of two types of actuators is used including heat actuators and piezoelectric actuators. With heat actuators, a heater, placed at a convenient location, heats the ink. This causes a quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, an electric field is applied to a piezoelectric material possessing properties that create a pulse of mechanical movement stress in the material, thereby causing an ink droplet to be expelled by a pumping action. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate.
The volume of ink ejected by such nozzles is determined by the quantity of fluid ejected at each actuation of the drive mechanism, the velocity with which the fluid is ejected, and the rate of ejection. For a given geometry of the chamber, the pressure at which the fluid is supplied to the chamber and the operational characteristics of the drive mechanism determine all of those parameters. By increasing the supply pressure and the displacement of the drive mechanism in the forward stroke, either independently or as combined parameters, the ejection quality can be increased. However, if the supply pressure is to be increased substantially above the pressure at the outlet of the jet (which in printheads is generally atmospheric pressure), the fluid column cannot be contained in the chamber during the off periods of the dispenser i.e. during periods when no fluid is to be ejected from that particular jet. Fluid will therefore drip out of the jet during those periods. Hence, the most influential parameter in achieving high-quality drop on demand in these known dispensers is the maximum obtainable displacement of the drive mechanism, which is clearly limited.
The second technology, commonly referred to as continuous stream or continuous ink jet printing, uses a pressurized ink source for producing a continuous stream of ink droplets from each ejector. Typically, the pressurized ink is in fluidic contact with all the ejectors through a common manifold. The energy to propel droplets from the ejectors comes from the pressurization means pressurizing the manifold, which is typically a pump located remotely from the printhead. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the ink droplets are deflected into an ink-capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or discarded. When printing is desired, the ink droplets are not deflected and allowed to strike a print media. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism.
Other methods of continuous ink jet printing employ air flow in the vicinity of ink streams for various purposes. For example, U.S. Pat. No. 3,596,275 issued to Sweet in 1978 discloses the use of both collinear and perpendicular air flow to the droplet flow path to remove the effect of the wake turbulence on the path of succeeding droplets. This work was expanded upon in U.S. Pat. No. 3,972,051 to Lundquist et al., U.S. Pat. No. 4,097,872 to Hendriks et al. and U.S. Pat. No. 4,297,712 to Lammers et al. in regards to the design of aspirators for use in droplet wake minimization. U.S. Pat. No. 4,106,032, to Miura and U.S. Pat. No. 4,728,969 to Le et al. employ a coaxial air flow to assist jetting from a drop-on-demand type head.
While this method does not rely on electrostatic means to affect the trajectory of droplets, it does rely on the precise control of the break off points of the filaments and the placement of the air flow intermediate to these break off points. Such a system is difficult to control and to manufacture. Furthermore, the physical separation or amount of discrimination between the two droplet paths is small further adding to the difficulty of control and manufacture. As such, these printheads suffer from a lack of precise control of the placement of drops on the print medium, which can produce visible image artifacts.
One problem associated with ink jet printers in general and such printers employing gas or air flows in particular is the drying of the ink. Ink drying in the vicinity of the printhead nozzles can lead to spurious droplet trajectories and nozzle clogging. In some cases, the evaporation of volatile ink solvents from the droplets as they fly through the air can increase the viscosity of the ink captured by the gutter, thereby causing difficulties during the ink recycling operation when the recycled ink is passed through a filter. This last problem becomes particularly difficult if the loss of solvent in the ink is large enough to cause the pigments in the ink to coagulate. Yet another problem associated with the guttering of inks is that the gutter is provided with a negative pressure, and is thereby subject to sucking wind, dirt, and frothy mist into the ink to be recycled.
European Patent Application No. EP-A-0436509 describes a fluid dispenser comprising a main chamber to which fluid is fed under pressure and a pair of outlet channels. A dispensing outlet channel leads to a dispensing outlet, whilst a recirculation outlet channel conducts the fluid back into the fluid supply. In use, the fluid normally veers towards the recirculation outlet channel leading back to the fluid supply. When a drop of fluid is to be dispensed, a driver device is momentarily energized so that the fluid flow switches over to the dispensing outlet channel. As soon as the required quantity of fluid has been dispensed, the flow is switched back to the recirculation channel by energization of a second driver device, so that the fluid again circulates back to the fluid supply. A disadvantage of the fluid dispenser is that two driver devices are required at each nozzle. Another disadvantage is that each nozzle requires a large footprint on the printhead to accommodate the pair of driver devices.
WO 95/10415 discloses a fluid dispenser comprising a supply channel; fluid supply means for feeding said main fluid to the supply channel under pressure; a first fluid path along which the main fluid is fed from the supply channel; a second fluid path including a fluid dispensing outlet; a control channel containing control fluid and having a control outlet adjacent the first fluid path, and means for changing pressure in said control fluid such that a wave front is formed in the main fluid and a droplet of said main fluid is dispensed from the fluid dispensing outlet. The main fluid flow follows the first fluid path due to Coanda effect except when diverted by change of pressure of the control fluid. While this fluid dispenser overcomes the need for two driver devices in European Patent Application No. EP-A-0436509, droplets to be dispensed are unsupported as they depart from the main fluid flow to exit the fluid dispensing outlet. As such, these printheads suffer from a lack of precise control of the placement of drops on the print medium, which can produce visible image artifacts. U.S. Pat. No. 4,345,259 is quite similar to WO 95/10415, and is cited here for the sake of completeness.